docs: def png(contents, latex_command):
      """Creates a png if needed."""
      outfile = sha(contents + latex_command)
      src = os.path.join(IMAGEDIR, outfile + '.png')
      if not os.path.isfile(src):
          try:
              os.mkdir(IMAGEDIR)
              stderr.write('Created directory ' + IMAGEDIR + '\n')
          except OSError:
              pass
          latex2png(latex_command + "{" + contents + "}", outfile)
          stderr.write('Created image ' + src + '\n')
      return src